Order in the periodic table — Measurement of ionization potentials of heavy elements confirms: actinide series ends with lawrencium

An international group of researchers including participants from GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt and its two branches, the Helmholtz Institutes Mainz and Jena, have determined the first ionization potentials of the artificially created elements fermium, mendelevium, nobelium, and lawrencium. The data unambiguously show that the actinide series ends with lawrencium. The results have been published in the Journal of the American Chemical Society (JACS).

The chemical elements fermium, mendelevium, nobelium, and lawrencium have the atomic numbers 100 to 103 in the periodic table of the elements. They do not occur naturally on earth, but can be artificially created, in nuclear fusion reactions at particle accelerators, for example. This process features low rates of production — at most a few atoms per second. All are unstable, decaying again within seconds to minutes. This renders studies of their chemical properties difficult, requiring complex experimental investigations of individual atoms.

In the current experiments the scientists looked at the first ionization potentials of the elements. This quantity measures the energy required to remove the least tightly bound electron from the outer shell of a neutral atom. The researchers expected the ionization potential to increase until nobelium is reached, which would correspond to a completely filled electron shell. For the following element, lawrencium, which possesses only a single, less strongly bound electron, a decrease in the ionization potential was expected.

Corresponding values for nobelium and lawrencium were already available from previous experiments. The present work expands the data set to the heaviest four members of the actinide elements, thus completing the data set of 14 elements of the whole actinide series. “The measured values are in agreement with the predictions of current relativistic calculations that were carried out in parallel with the experiment, and with the measurements carried out on nobelium using laser spectroscopy by a further collaboration working at GSI,” explained Professor Christoph Düllmann, head of the Superheavy Elements Chemistry departments at GSI and the Helmholtz Institute Mainz. “With this experiment we were able to unequivocally demonstrate that the actinide series ends with lawrencium, in analogy to the lighter lanthanide series, which is located above the actinides in the periodic table.”

The researchers were able to create and measure the artificial elements at the Tandem accelerator and the attached isotope separator at the Japanese research organization JAEA in Tokai, Japan. The first ionization potentials were determined using a surface ionization process. A gas stream in a Teflon tube carried the elements to a tantalum chamber with a surface heated to up to 3,000°C, where they could be ionized. Comparing the number of atoms fed in with that of ionized atoms provided a value for the efficiency of the ionization, from which the first ionization potential of the elements could be determined.

Research institutes from Germany, the Netherlands, Japan, Israel, and Switzerland participated in this work. (cp)